Personally, I think that you have the best size for your Zetec with SVT wheels. That was the stock size for the 98 SVT.
The 99 and 00 used 215/50R16. This tire is slightly wider and slightly smaller in diameter.
Many are using 225/50R16. This size is, as you noted, nearly an inch wider, and close to the same diameter. Because the optimal wheel width for this size is 7" and the SVT wheel is 6.5", the actual diameter will be very imperceptably larger. This size is a very close fit. Clearance is very close, but adequate. The larger footprint of this size will make the car slightly less "tossable" in handling, and feel a little heavier although you can actually push through a corner faster. I think your 4 cylinder will handle better with the 205/55/R16.
Some have also used a 225/45R16. I have no experience with this size on our cars.
